CAPTCHA A CAPTCHA is a type of challenge – response Test that response is generated by a human being or computerCAPTCHA – Completely Automated Public Turing test to tell Computers &Humans Apart.
Why captcha was needed? Online Polls Protecting Web Registration E–Ticketing Email Spam
Turing Test :- To test a machine’s level of intelligence CAPTCHA employs a reverse Turing testjudge = CAPTCHA programparticipant = userif user passes CAPTCHA, he is humanif user fails , it is a machine
Types of CAPTCHAs :-Texts CAPTCHAsGimpyEz – GimpyMSNs Passport Service CAPTCHAsAudio CAPTCHAs
Texts CAPTCHAs :-Simple, normal questions like: What is twenty minus three? What is the third letter in UNIVERSITY? If yesterday was a Sunday, what is today?
Gimpy :- Designed by Yahoo & CMU(Carnegie Mellon University) Picks up 10 random words from dictionary, and displayingthem in a distorted and overlapped manner. User has to recognize at least 3 words. If the user is correct, then he is admitted.
Ez – Gimpy :- A modified version of Gimpy. Yahoo used this version in Messenger. Randomly picks a single word from a dictionary and appliesdistortion to the text.
MSNs Passport Service CAPTCHAs :- Provided for Microsoft’s MSN services. Eight characters(upper case) and digits. Warping is used to distort. Very strong implementation, hasn’t been broken.XTNM5YRE L9D28229B
Audio CAPTCHAs :- Consists of downloadable audio clip. User listens & enters the spoken word. Google’s audio enabled CAPTCHA
reCAPTCHA :-Used in Google books project.Two words are shown, the program knows the first word.If the user enter the first word correctly, it assumes that the second unknownword will also be entered correctly.